rust: bindgen requires rustfmt

Bindgen will use rustfmt after generating the bindings, but this will
fail if rustfmt is not installed. Only run bindgen if rustfmt is
installed.

configure.ac

@@ -2178,6 +2178,9 @@ fi
     AC_SUBST(rust_vendor_comment)
     AM_CONDITIONAL([HAVE_RUST_VENDOR], [test "x$have_rust_vendor" = "xyes"])
 
+# Bindgen requires rustfmt.
+    AC_PATH_PROG([RUSTFMT], [rustfmt], [no])
+
 # Check for bindgen
     AC_PATH_PROG([BINDGEN], [bindgen], [no])
     if test "x$BINDGEN" != "xno"; then
@@ -2194,7 +2197,7 @@ fi
         BINDGEN="no"
       fi
     fi
-    AM_CONDITIONAL([HAVE_BINDGEN], [test "x$BINDGEN" != "xno"])
+    AM_CONDITIONAL([HAVE_BINDGEN], [test "x$BINDGEN" != "xno" -a "x$RUSTFMT" != "xno"])
 
     have_rust_headers="no"
     AC_MSG_CHECKING(for $srcdir/rust/dist/rust-bindings.h)

rust/Makefile.am

@@ -119,7 +119,7 @@ if HAVE_BINDGEN
 		echo "Not checking bindings for out of tree build"; \
 	fi
 else
-	@echo "Unable to check bindgen bindings: bindgen not found"
+	@echo "Unable to check bindgen bindings: bindgen or rustfmt not found"
 endif
 
 check:
@@ -165,7 +165,7 @@ if HAVE_BINDGEN
 	cat sys/src/jsonbuilder.rs.tmp >> sys/src/jsonbuilder.rs
 	rm -f sys/src/jsonbuilder.rs.tmp
 else
-	@echo "error: bindgen not installed, can't update bindings"
+	@echo "error: bindgen or rustfmt not installed, can't update bindings"
 	exit 1
 endif